At the present electrical machines comprise one or more electrical conductor windings each of which has a polymeric insulation material. These electrical machines have a maximum operating temperature of the order of 200° C., due to the polymeric insulation material applied to the electrical conductor windings.
The use of specialist high temperature polymeric insulation material would enable the electrical machines to have a maximum operating temperatures of the order of 250° C. However, it is believed that stable, oxidation resistant, polymeric insulation material will not have maximum operating temperatures above 300° C.
There is a requirement for electrical machines with maximum operating temperatures of the order of 500° C. and above. These electrical machines for example are active electromagnetic bearings, electrical generators and electrical motors for use within gas turbine engines, particularly on the high-pressure shaft/rotor of a gas turbine engine. The use of active electromagnetic bearings may allow the simplification of the gas turbine engine by elimination of conventional bearings and oil systems.
As discussed above polymeric insulation material cannot be used at temperatures above about 300° C.
The use of an inorganic insulation material for the electrical conductors is a possibility. The inorganic insulation material may be based on ceramic cloths or ceramic coatings, applied to the electrical conductors. However, this is not desirable because the inorganic insulation material tends to be bulky limiting the packing density of the electrical conductor and the electrical conductors require potting in an inorganic cement. Additionally the manufacturing process is very labour intensive.
The use of an inorganic insulation material may be based on ceramic cloth and inorganic cement. However, this is not desirable because these inorganic insulation materials have poor thermal conductivity and would make the thermal management of the electrical conductor difficult. Additionally the porous nature of the inorganic cement tends to allow the inorganic insulation material to soak up fluids, for example water, oil or other lubricant. The presence of moisture tends to degrade the electrical insulation by allowing leakage currents to earth or between turns of the electrical conductor. The presence of oil tends to degrade the electrical insulation by forming carbon also allowing leakage currents to earth or between turns of the electrical conductor. Also the thermal expansion mismatch may cause damage to the insulation material during thermal cycling of the electrical conductor.